Low latency and high reliability have always been the key requirements for industrial applications such as factory automation, power and energy system monitoring and control, smart metering, and oil and gas businesses. In the past, such industrial control systems have included a large number of cables in order to connect, monitor and control the array of devices and sensor units across an industrial plant. It can be quite expensive to install, operate and maintain such a wired system. Despite these disadvantages, the industry is still reluctant to switch to cheaper wireless internet of things (loT) solutions which have been on the market for the past few years.
The IEEE 802.15.4 standard specifies the physical layer and media access control (MAC) layer for low-rate wireless personal area networks, such as those used in industrial applications. Previous IEEE 802.15.4 standard solutions, whilst being cheaper, have not been able to deliver the desired low-latency and high reliability requirements due to their carrier sense multiple access with collision avoidance (CSMA/CA) based contention MAC access methods.
Time Division Multiple Access (TDMA) scheduling is an alternative to CSMA/CA. TDMA splits time into small intervals called timeslots. Each individual node in the network is assigned with a schedule (a set of timeslots), in which they can transmit data on a channel (e.g. a certain frequency bandwidth) without collision with other transmissions. Since TDMA is contention-less and collision free, it can achieve much lower multi-hop latency and higher communication reliability compared with CSMA/CA based approaches. Having said this, it still suffers from multi-path fading and interference from other devices in the Industrial, Scientific and Medical Radio (ISM) band such as WiFi, Bluetooth and microwave devices, to name a few.
To overcome these limitations, the Time-Slotted Channel Hopping (TSCH) MAC provides a promising way to enable deterministic mesh networking and paves the way for future low-power wireless industrial applications (the Internet of Important things). TSCH achieves improved communication reliability via channel hopping and avoids external interference operating at the same frequency band. Channel hopping helps to spread the risk of collisions by periodically switching the channel over which transmissions are made. TSCH also maintains low radio duty cycle and low energy consumption by synchronizing network nodes.
The Internet Protocol version 6 over the TSCH MAC (6TiSCH) standard is a developing Internet Engineering Task Force (IETF) standard. Along with a scheduling mechanism to enable deterministic wireless communication, it can provide ultra-low latency and high reliability guarantee for industrial applications.
Conventionally, in a 6TiSCH network, to ensure a deterministic communication, each scheduled communication link has one transmitter and one receiver. In addition, there is a dedicated routing path and each communication link along the routing path needs to be scheduled according to the multi-hop link traffic. The scheduler therefore needs to decide explicitly which node should transmit or receive on which channel at which timeslot.
The general rule is that a node should not transmit or receive at the same time or receive multiple messages designated to it at the same time. Secondly, interference may also occur when multiple communications take place at the same time within interference range of each other. Scheduling is therefore used in order to avoid all such conflicts.
Having said this, scheduling can be a rather complex process, which can incur additional delays and consume more resources for control overheads. This can also impose several limitations. A network with a tightly scheduled communication can be especially vulnerable to network dynamicity such as variable traffic rates, node failure and node mobility.
There is therefore a need to improve scheduling and provide spatial diversity to the TSCH MAC standard, which can greatly improve communication reliability and significantly reduce the re-scheduling complexity even when the nodes are mobile.